Python: Change the texture resolution limit individually ? Per Material?
you mean independently from the simplify ? just by changing image size ? If you are looking in to this I have a small experience since I did something similar before few years in order to bake textures and export them for webgl (DAE) with the minimum res I could afford for online apps.

there is a scale property in bpy.data.image object, according to this thread it must be relatively easy to implement it, although from a quick search I found out that this is only for new made images created in blender. 

Thus for you will need an external module to accomplish it, so take a closer look on the second response which is mentioned also as the solution.

How to change image dimensions of a generated image from Python?